This invention relates to plastic material and more particularly to the removal of moisture and volatiles from the plastic material.
In the injection and extrusion molding fields, particulate plastic material is normally fed to an elongated screw which is rotated in a heated barrel to plasticize the material to form melt. Well known and difficult problems in these fields are the removal of moisture from the particulate plastic material before entering the barrel and the removal of volatiles which are produced during plasticizing. While the type and volume of volatiles produced varies with the type of material and the particular application, this term is generally used herein to refer to the vaporized moisture, additives, binders, softeners, lubricants and other materials given off as a gas during plasticizing. As is well known, the volatiles often include monomers or other materials which are toxic or otherwise undesirable in the working environment. If sufficient moisture and volatiles are not removed, then the products molded from the melt frequently contain bubbles or other unacceptable imperfections.
In regard to the moisture problem, the particulate plastic material is often hygroscopic and it is normally shipped and stored in sealed containers. Nevertheless, it has remained necessary to dry the material prior to use to produce a satisfactory product. This is often done by heating the material in dryers for several hours using elaborate equipment. Various apparatus such as drying and vacuum ovens and desiccant filters have been designed for this purpose. One early type of continuous dryer for grain is shown in U.S. Pat. No. 759,527 which issued to Irwin May 10, 1904. However, drying has remained a requirement which is both costly and inconvenient due to the amount of electrical power necessary and the additional equipment required.
In regard to the volatiles problem, various provisions have been made to vent them from the barrel in which the screw rotates. However, this has the difficulty that the heat and the shearing forces produced by the screw that gradually melts the particulate plastic material and releases the volatiles causes problems with the vent. U.S. Pat. No. 3,655,313 to Kunogi which issued Apr. 11, 1972 discloses apparatus which controls the feed of particulate plastic material to the barrel. This structure has the disadvantage that it is difficult for the volatiles to escape. A later U.S. Pat. No. 3,826,477 to Kunogi et al. which issued July 30, 1974 discloses a relatively complicated combination of various barrel internal diameters and screw outer diameters to provide for a separate opening to which suction is applied. Both of these patents were assigned to Meiki Co., Ltd. which has a brochure entitled "M-Series Injection Molding Machine" No. C-320B 83. 8. 700 A which shows the use of a controlled feed with a separate vent opening. Providing a separate opening for the removal of the volatiles has the disadvantages that a longer more expensive screw is required and no heat is recovered from the hot volatiles passing through the incoming particulate plastic material.
More recently, as described in U.S. patent application Ser. No. 517,580 filed Mar. 16, 1982, now abandoned, the present inventor provides an earlier method and apparatus for the removal of the moisture and the volatiles without the necessity of pre-drying. This similarly involves applying suction to remove the volatiles through the opening in the barrel, and while this has worked very successfully for some applications, it has continuing problems for other applications. For instance, in some applications, the volatiles appear to have difficulty in flowing along the barrel to be removed through the opening. In large volume applications, plasticizing produces a considerable quantity of heat as well as moisture which tends to partially melt the particulate material in the area of the barrel opening causing it to bridge over and eventually restrict flow. In smaller applications, the mere presence of the suction tube in the opening can hinder flow of the particulate material. Furthermore, in injection molding applications, the normal process of the particulate material graudally melting as it is carried along the screw is disrupted when the screw is longitudinally actuated in the barrel to inject the melt. This causes the particulate material carried forward by the screw to be covered by the existing melt before the volatiles can be released. Another disadvantage of this previous device is that particles of plastic material are picked up by the hot gases and carried into the suction pipe at the material inlet. These airborne particles circulate in the interior of the preheater guide member and often stick together and to small particles from the incoming particualte material to form a cigar-shaped body which gradually builds up until it plugs the suction system.
In the present case, the inventor has found that a common opening in the barrel may be successfully used to both feed the particulate plastic material and remove hot volatiles by utilizing a combination of controlled feeding and the application of suction to remove the volatiles.